The acoustics of a room has a significant impact on an audience's perception of the quality of a live performance. There are a number of properties of rooms that have been identified as being correlated to subjective impressions of quality. The earliest measured parameter was the reverberation time. This is a global property of the room which has a similar value at all locations. It is governed by the room volume and the absorption of the room surfaces, and the quality of reverberation is also governed by the room shape. Rooms with a long reverberation time can provide a sense of envelopment which produces an increased enjoyment of performances such as opera or classical music. However, the same acoustics can reduce the intelligibility of the spoken word, and therefore be unsuitable for speech.
Other parameters have been determined which relate to the properties of the early part of the response, such as the clarity. More recent auditoria have been designed with reflectors specifically placed to enhance the early part of the room response to sounds emanating from the stage.
To achieve maximum enjoyment of a variety of performances, the acoustics of a room must be matched to the intended performance. Many rooms have for this reason been made acoustically adjustable. For example adjustable absorbers such as moveable curtains or rotatable panels have been used to control reverberation time. Extra acoustic spaces have been constructed which can be coupled to the main area when required to provide more reverberance.
Electroacoustic systems have been used for many years to enhance the acoustics of rooms. The simplest system is the public address or sound reinforcement system, in which the sound produced by performers on stage is detected by close microphones and the sound amplified and broadcast from one or more sets of loudspeakers. The goal of such systems is typically to project the direct, unreverberated, sound to the audience to eliminate the effects of the room and improve clarity.
More recently, more complex forms of sound system have been developed which aim to provide adjustable room acoustics. The basic sound reinforcement system has been further developed by introducing sound processing elements such as delays, which allow the creation of additional sound reflections—see W. Anhert, “Complex simulation of acoustic fields by the delta stereophony system (DDS),” J. Audio Eng: Soc., vol. 35, no. 9, pp 643-652, September 1987, and U.S. Pat. No. 5,142,586. The delta stereophony system described by Anhert provides sound reflections that are arranged to arrive later than the direct sound, in order to maintain correct localisation. For a given receiver location, the appropriate delays can be chosen to avoid preceding the direct sound, but the delays must be changed for different receiver positions. The ACS system described in U.S. Pat. No. 5,142,586 claims to provide reflections at the appropriate times for all receiver positions, by the creation of wavefronts. The delays are chosen using Huygens principle, and their quantification mathematically by integral equations is described by A. J. Berknout, D. de Vries, and P. Vogel, “Acoustic control by wave field synthesis,” J. Acoust Soc. Am, vol. 93, no. 5, pp 2764-2778, May 1993. The wavefronts are generated using loudspeaker arrays. These electroacoustic systems offer a more controllable early reflection response than can be achieved using passive reflectors.
Reverberators have also been introduced to provide a larger reverberation time for sources on stage—see for example U.S. Pat. No. 5,109,419. Larger numbers of speakers have also been employed to provide enhanced reflections and reverberation, such as to under balcony areas. The microphones have also been positioned further from the performers so as to be less obtrusive, while still aiming to detect the direct sound.
The systems discussed above avoid feedback from the loudspeakers to the microphones, since such feedback can lead to colouration and instability if the loop gain is too high. Because of this fact, they may be generically termed in-line, or non-regenerative, systems. Such systems can provide large increases in reverberation for sound sources that are close to the microphones (ie on stage), but they have a small effect for sound sources at other positions in the room.
A second type of enhancement system is the non-in-line, or regenerative, system, which seeks to utilise the feedback between loudspeakers and microphones to achieve a global enhancement of reverberation that occurs for any sound source position—see A. Krokstad, “Electroacoustic means of controlling auditorium acoustics,” Applied Acoustics, vol. 24, pp 275-288, 1998 and F. Kawakami and Y. Shimizu, “Active field control in auditoria,” Applied Acoustics, vol. 31, pp 47-75, 1990. Since the natural, unassisted reverberation time is largely the same for all source positions, the regenerative systems can provide a more natural enhanced reverberation. Non-in-line systems typically use a large number of independent microphone, amplifier, loudspeaker channels, each with a low loop gain. Each channel provides a small enhancement of reverberation at all frequencies, with low risk of colouration, and the combined effect of all the channels is a significant increase in reverberation and loudness. The microphones are positioned in the reverberant field from all sound sources in the room to ensure that the system produces a similar enhancement for all sources. Non-in-line systems, however, have typically required from 60 to 120 channels, and have therefore been expensive. Furthermore, since the microphones are remote from all sources, they are less suited to providing significant early reflections than in-line systems.
More recently, a non-in-line system has been developed which uses a multichannel reverberator between the microphones and loudspeakers to provide an increase in reverberation time without requiring an increase in loop gain—see U.S. Pat. No. 5,862,233. It has been shown that the system can both reduce the apparent room absorption (by increasing the loop gain) and increase the apparent room volume (by increasing the reverberation time of the reverberator)—see M. A. Poletti, “The performance of a new assisted reverberation system,” Acta Acustica, 2 Dec. 1994, pp 511-524. In general, a hybrid room enhancement system may be constructed in which some of the microphones of a non-in-line system containing a reverberator are moved close to the source. In this case the system demonstrates properties of both in-line and non-in-line systems—see M. A. Poletti, “The analysis of a general assisted reverberation system,” accepted for publication in Acta Acustica vol. 84, pp 766-775, 1998.
When used solely for early reflection enhancement, an in-line system provides a finite number of delayed outputs to simulate early reflections. However, if operated at moderate to high gains, the system runs the risk of instability. This is particularly likely if new delays/reflections are added which will increase the loop gain at some frequencies.
In any sound system, it is important that the direct acoustic sound from the stage arrives at every member of the audience before (or at the same time as) any electroacoustic signal. This is because the perception of localisation is governed by the first signal to arrive at the ears (provided later signals are not overly large). Hence, care must be taken in both in-line and non-in-line systems to ensure that the electroacoustic signals are suitably delayed. In a non-in-line system this can be achieved by keeping the microphones a suitable distance from the stage. Delays can be used in in-line systems and non-in-line systems to avoid preceding the direct sound. Care must therefore be taken in any non-in-line system where microphones are moved close to the stage.